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Continuous footings by the foot, or square pads by the count. Get cubic yards with waste and the order to give the plant.
ConCal takes the slab, footings, steel and labor from one drawing and turns them into a bid your customer can sign.
A footing is a long box. Multiply the length by the width by the depth, all in feet, then divide by 27 to get cubic yards.
Cubic yards = run (ft) × width (in ÷ 12) × depth (in ÷ 12) ÷ 27
Worked example. A 30 by 30 garage has 120 feet of footing. At 20 inches wide and 10 inches deep: 120 × 1.667 × 0.833 = 166.7 cubic feet. Divide by 27 and you get 6.17 yards. Add 10% waste and you get 6.79, so order 7.
For pad footings under posts or columns, figure one pad the same way and multiply by the count. Six pads at 24 by 24 by 12 inches are 4 cubic feet each, 24 cubic feet total, or 0.89 yards before waste.
A slab is formed, so its edges are where you set them. A footing is often poured right in a dug trench, and a trench is never as neat as the plan. A trench dug 2 inches wide on a 20 inch footing is 10% more concrete on its own. That is why 10 to 15% waste is common on footings.
Always use the footing size on the plans or what your local code calls for. The numbers in the boxes above are only a starting point.